It was, fashion aficionados said, a suspiciously svelte image of Demi Moore on the cover of the American magazine W. At first glance, the 47-year-old actress – encased in a gold corset – simply looked very thin. But as commentators and bloggers looked closer a consensus emerged: the photograph had been digitally altered.

Part of Moore's left hip appeared to have been airbrushed out, leaving a small section of flesh missing, and her thigh seemed wider than her hip. But the star of GI Jane and Striptease then went on Twitter to post what she said was the original photo, and denied it had been heavily doctored. She wrote in her blog: "Here is the original image – people, my hips were not touched." The photographers Mert Alas and Marcus Piggott also denied altering their picture of Moore.

Now the controversy has taken a new turn after it was suggested that, rather than digitally altering Moore's photo, W's art directors simply superimposed upon it an image of a Polish model, Anja Rubik, wearing the same outfit.

The photos appear to match almost perfectly, and a Los Angeles photographer, Anthony Citrano, has offered to give $5,000 to charity if Moore can prove the cover shot was not radically altered. The actress and W magazine have yet to respond.

It is not the first time that alleged digital manipulation of fashion photos has caused controversy. In October, Ralph Lauren was forced to apologise after transforming the healthy frame of model Filippa Hamilton in an advertisement for its Blue Label Jeans. Her waist was digitally reduced so that it appeared improbably tiny. In 2003, GQ admitted altering a cover portrait of Kate Winslet in a leotard so that she looked much leaner.

Retouching is an open secret in fashion and advertising, and almost every image that appears in a magazine will have been enhanced in some way, such as to make skin tone more even or remove dust speckles from a picture. A recent issue of Elle revealed how a cover photo of Kate Hudson was doctored to remove shadows. The 20th-century photographer Cecil Beaton would routinely alter his society portraits to flatter his subjects.

Belinda Coleman, manager of the London-based retouching agency The Shoemakers Elves, said: "As a general rule, almost every image has been altered in some way. It is just a question of degrees. People can become addicted to retouching, however.

"It's there to enhance photography, not alter it to become something completely different."

Drastic manipulation of images, such as that allegedly carried out by W, "does not happen very often", Ms Coleman said. But she added: "Without seeing the original image it is impossible to tell what they did."